In a convertible perfecting printing press, the sheet to be printed is transferred between a first and second individual printing unit via a storage cylinder having twice the normal diameter and a turning cylinder. Means are provided for converting the storage cylinder and turning cylinder from a first form or a single-sided printing mode to a perfecting or double-sided printing mode, and means are also provided to accommodate different format sizes of the printed sheets. For perfecting, the sheet to be printed is guided by the storage cylinder with the leading edge of the sheet conveyed passed the tangential point between the storage cylinder of double diameter and the subsequent turning cylinder. As soon as the trailing edge of the sheet, which is held by a suction or calendering means, reaches the tangential point, it is gripped by turning grippers of the turning cylinder. The release of the leading edge of the sheet from the storage cylinder grippers occurs at the same time and the sheet is transferred with its originally trailing edge in leading position to the second printing unit.
Changeover of the storage cylinder and the turning cylinder from perfecting to first form printing or vice versa requires adjustment of the relative phase between the cylinders and also adjustment of the cams which control operation of the storage cylinder grippers and the turning grippers. Changeover is further complicated by adjustment of the printing format since the phase between the storage cylinder and the turning cylinder for perfecting is dependent upon the format adjustment. Due to these multiple adjustments, the changeover operation is highly complex and must be performed reliably if faulty printing, mistakes in sheet conveyance and damage to the printing press are to be avoided. Design of the changeover mechanism has been given considerable attention as is evident from Becker U.S. Pat. No. 4,014,261 issued Mar. 29, 1977. Moreover, interlocks have been provided so that the printing press can be cleared for operation only after completion of changeover and only after all the changeover steps have been carried out correctly.
A known interlock system for a changeover mechanism is disclosed in West German Pat. No. 2,620,392 and uses electromechanical devices. During adjustment of the phase between the storage cylinder and the turning cylinder, the turning cylinder is prevented from being turned by a locking lever engaging in a locking pin on the turning cylinder drive wheel. Adjustment of the cam follower rollers of the turning grippers is sensed by limit switches being abutted in the end positions. Adjustment of the storage cylinder cams is controlled simultaneously with rotation of the storage cylinder by a toothed segment whose rotation is secured electromechanically and in dependence upon gripper control. Limit switches are provided for detecting the position of the turning and storage cylinder gripper controls and for detecting actuation of the adjusting means and for securing the turning cylinder in rotation, and these limit switches are connected in series and logical AND-relationship to interrupt the power supply to the printing machine in the absence of coincidence.
The electromechanical interlock system disclosed in West German Pat. No. 2,620,392 has several disadvantages in its association of the various mechanical means to detect adjustment positions. The storage cylinder cam must be adjusted by a complete cam disk rather than a toothed segment in order to provide sensing surfaces for the necessary limit switch. Lever mechanisms are needed to protect the turning cylinder and its gripper control. More importantly, access to the changeover means is not adequately protected to prevent adjusting operations from being carried out unchecked and at inappropriate times. There is no way to ensure that the printing units are properly coupled after changeover and the gearing is reengaged, especially when the storage cylinder has been adjusted for a new printing format.